Conventionally, as methods for protecting such systems from execution of illegal and user not-intended programs, a first protection method (refer to, for example, non-patent publication 1) and a second protection method (refer to, for instance, patent publication 1, or patent publication 2) are known, while these systems contain memories capable of deleting and rewriting programs stored thereinto, and CPUs for interpreting instructions of the programs stored in the memories so as to execute the interpreted instructions. In the first method, since a rewriting operation of a memory for storing thereinto a program to be executed is protected by a predetermined method, a rewriting operation of an illegal program is prevented and the execution of the illegal program is prevented. In the second method, a program stored in a memory is checked by way of a predetermined method after a power supply of a system has been turned ON, or hardware has been reset, and then, only such a program which is judged as a normal program is executed by a CPU, so that the execution of the illegal program can be prevented.
As the first method, for instance, while the protection function is known which is installed in the flash memory S29GL512N manufactured by FASL LLC firm, a permanent protection for protecting a system from illegal rewriting operation by executing a predetermined command, a password protection for protecting a system from an illegal rewriting operation based upon a password defined by a user, and the like are known.
As the second method, the inventive idea described in the patent publication 1 is given as follows: That is, in the apparatus containing: the external ROM for storing thereinto the program used to operate the system; the built-in PROM constituted in such a manner that after the information is written thereinto, the written information cannot be deleted and rewritten; and the CPU, after the power supply of the system is turned ON, or the hardware is reset, the data of the external ROM is checked based upon the program stored in the built-in RROM. When the data of the external ROM is normal, the program of the external ROM is executed, whereas when the data of the external ROM is abnormal, the checking operation is repeatedly carried out.
Also, as the second method, the inventive idea described in the patent publication 2 is given as follows: That is, in the apparatus containing: the memory for storing the execution program which has been message-digest-processed; and the processing unit for performing the message-digest-process operation and for executing the program, after the power supply is turned ON, or the hardware is reset, the predetermined execution program is checked by way of the message digest method. When the execution program is normal, the execution program is executed, whereas when the execution program is abnormal, the execution program is stopped.    Non-patent publication 1: S29GLxxxN MirrorBit flashfamily data sheet Document No. J27631, Revised A, Ammended I, issue date: Dec. 15, 2003 (27631-AI Oct. 16, 2003), page 45 sector protect Patent publication 1: JP-A-9-325914    Patent Publication 2: JP-A-11-39158.